This is according to a September 2016 FBI report. The report indicates that Clinton and her immediate staff were repeatedly “notified of foreign travel risks and were warned that digital threats began immediately upon landing in a foreign country, since connection of a mobile device to a local network provides opportunities for foreign adversaries to intercept voice and email transmissions.”
Additionally, the State Department has a Mobile Communications Team responsible for establishing secure mobile voice and data communications for Clinton and her team wherever they travel. But even so, Clinton and her staff frequently use their private and unsecure mobile devices and private email accounts while overseas.
The number of Clinton emails sent or received outside the US will be redacted in the FBI report. Although it will mention that “hundreds” were classified at the “confidential” level, additional details are redacted. Nearly all mentions of “top secret” emails are redacted in the report, so it’s impossible to know if any of those are sent while Clinton is overseas.
The report will mention that some emails between Clinton and President Obama are sent while Clinton is overseas. However, the exact number will be redacted. None of these overseas emails between them will be deemed to contain classified information. According to the report, “Clinton told the FBI that she received no particular guidance as to how she should use President Obama’s email address…”
The details of the FBI’s report on Clinton’s July 2016 FBI interview will indicate that Clinton emailed Obama on July 1, 2012 from Russia. However, it is not clear if she sent the email from on the ground or on a plane. (Federal Bureau of Investigation, 9/2/2016)